What is a plural? 1 file, 2 files, 3 files: file is singular, files plural.
In this simple example we see the English plural form. Other languages
have simpler or even more elaborate plural forms.
Both KDE and Gettext have a method of managing plurals for PO files so that
plurals can be used in languages that do not follow the English convention.

Well the simple answer is that not all languages follow the pluralisation
format of English. Some use the singular form for both 0 and 1. Others have
no singular/plural form. And yet others have complex systems that have at
least 4 plural forms. For English speakers think of it as the different
suffixes you see here: “1st, 2nd, 3rd, 4th … 21st, 22nd, 23rd, 24th…”

So in order to cater for the various plural forms a system was developed.

Also in Gettext style plurals the PO file header has an entry for the plural
form which defines the number of plurals as well as the mathematical function to
determine which one to use depending on the input number. Here is an example
which would work for English: